Transistor power dies are conventionally bonded to a metal substrate of aluminum, for example, by interposing, between the aluminum substrate and the die, a dielectric layer and possibly other layers of various materials. The other layers are typically selected to permit the die to be soldered to the layered stack of materials so as to maximize heat transfer between the die and the substrate without introducing materials that are incompatible with the die in terms of coefficient of expansion.
In a typical application where the metal substrate is aluminum and the dielectric consists of a layer of alumina (AL.sub.2 O.sub.3) on the substrate, a metal layer of nickel or copper interposed between the silicon die and the alumina would provide a surface to which the die could be readily soldered. However, the coefficients of expansion of nickel and copper are so different from that of silicon that the thermal stresses would be unacceptable in some applications. Other metals such as molybdenum and tungsten have an acceptable coefficient of expansion and exhibit good thermal and electrical conductivity. The latter metals, however, are not readily solderable to silicon.
Various attempts have been made to find economical techniques for bonding a silicon die to an aluminum substrate while taking into account the above-mentioned characteristics of the materials. See, for example, U.S. Pat. No. 4,542,401. Nevertheless, there is still a need for a reliable and economical arrangement for heat sinking a semiconductor die to a substrate so that it can withstand the thermal extremes and other stresses which are found in harsh environments such as under-the-hood automotive applications.
Accordingly, it is a general object of the invention to provide an improved technique for bonding a semiconductor die to a metal substrate, and also to provide an improved semiconductor arrangement which is capable of withstanding relatively extreme thermal shocks.
It is another object of the invention to provide the above-mentioned improvements for a semiconductor power die so as to permit the die to be easily soldered and yet avoid the thermal stresses commonly encountered with typical solder-receptive materials.